[http-server-csharp]: Minor improvements for sample generation

[markcowl]

• Emit a simple action for GET requests with a required or optional body: using standard request binding will fail at runtime for such cases. Also, emit a warning as this is generally a speec mistake
• Initialize enum values in mocks
• Add an HttpContextAccessor to standard DI container, and use in mock controller constructor, with documentation (used to change response status code, response properties, get http authentication info)
• Fix nullability of object in optional properties (Union types)